Electrolysis of alkali metal chlorides

ABSTRACT

Disturbances occurring in chloride-caustic cells using the amalgam method are caused by felty coatings which form on the bottom of the cells. These disturbances can be avoided by treating the mercury (after it has left the decomposer and before or after it has entered the electrolytic cell) with an aqueous solution which contains chlorine and/or ferric chloride.

United States Patent n91 Opp et al.

[ ELECTROLYSIS OF ALKALI METAL CHLORIDES [75] lnventors: Karl Opp,Heidelberg; Ewald Wygasch, Ludwigshafen; Gotthard Csizi, Bad Duerkheim,all of Germany [73] Assignee: Badische Anilln & Soda-FabrikAktiengesellschaft, Ludwigshafen/Rhein, Germany [22] Filed: Apr. 21,1971 [21] Appl. No.: 136,213

[30] Foreign Application Priority Data Apr. 27, 1970 Germany P 20 20480.2

[52] U.S. Cl 204/99, 204/140, [51] Int. Cl ..C0ld 1/08 C23b/-l/00 [58]Field of Search 204/99, 128, 140

[4 1' Aug.2 8, 1973 [56] References Cited UNITED STATES PATENTS3,639,118 2/1972 O'Grady 204/140 X 3,364,128 l/l968 Williston et a1204/140 Primary Examinr.lohn H. Mack Assistant Examiner-D. R. ValentineAttorney-Johnston, Root, O'Keeffe, Keil, Thompson & Shurtleff 57ABSTRACT Disturbances occurring in chloride-caustic cells using .theamalgam method are caused by felty coatings 4 Claims, No DrawingsELECTROLYSIS F ALKALI METAL CHLORIDES In the electrolytic productionof'chlorine and analkali metal hydroxide by the mercury method, cellsare used whose bottom consists of iron and whichamustbe coveredcompletely with mercury. In order to=achieve this, the bottom isamalgamated prior tosetting'the cell in operation so that it is madewettable by mercury; and complete coveringof the bottom'isachievedeven'wlien the layer of mercury isnot very thick. The coveringof mercury is essential during operation of the cell? because of therisk of therisk of the formation: of hydrogen.

Onthe other hand; for reasons of keeping the comsumptionof energy at. aminimum; the cell potential should be at an optimumelow, he. thedistance between* the anodes and the'mercury film shouldbe'assmall and'above all as uniform as possible. Itis-unfavor-able if; owing to localvariations in the thlCkHBSSlOf the mercury layer, the mean distancebetweenanode and cathode hasto bemarkedly increased. such' an unevendistribution of mercury may result in the mercury film being splitapartand thenhydrogen will: be formedas mentioned above.Inchlorine-caustic cells'as designed according to the present stateoftheartzthe two disturbances outlined above are caused by thedepositionof traces of iron inthe mercury. Iron'is oficourse'insoluble in mercurybut forms asuspensiom offine" crystals of iron in the mercury. Theseparticle'areretained in' the cell by magnetic forcessothatafterafewweeks? operation a felty coatingrforms on=the-bottom ofthecell.Short-circuits occur owing to the localretentionof' mercury thuscausedand'it'is'necessary to'raise the anodes. This increase in themeandistance between-the electrodes causes an-increase inthecell-potential. The latest remedy for this is-to open the. cellsatregularintervals-ofi to 8 weeksand toremove the. iron amalgammechanically fromthe bottom. This-operation notonly causes considerableassembly costs: but alsoconsiderable losses in output.

In order to obviate this difficulty, attempts have been made to incline.the cells more steeply in the hope that the mercury would flowsufficientlyrapidly to carry the iron with it. This hope has only beenpartly realized because the thin layer of mercury cannot be acceleratedbeyond a certain extent even at a very great inclination. Moreover themagnetic forces also become greater as the current densities areincreased. The said .efi'ect can be delayed by the said measures butcannot be prevented.

The present invention has as an objectthe prevention of the feltycoating on the bottom of the cells in'the electrolysis of alkali metalchloridesaccording to the amalgam process.

We have found that the said object is achieved in the electrolysis ofalkali metal chlorides by treating the mercury (after the alkali metalamalgam has been destroyed) with an aqueous solutioncontaining chlorineand/or ferric chloride.

Treatment of the mercury according to this invention is carried outafter it has left the decomposer and before it enters the electrolyticcell. or inside the latter. It may be carried out for example in packedtowers into which the two liquids (mercury and the solutioncontainingchlorine or ferric chloride) ar introduced. The treatment mayalso be carried out inside the electrolytic cell itself, for example byappropriate subdivision of the same.

Since intense mixing of the two liquids is" not neces sary for theremoval of iron from the mercury, how ever, it is possible according toa preferred erri'bodi'- ment of the invention to'carryoutthetreati'ne'rit" of thie mercury by providing a" covering of theaboveme'ri"-' aqueous solution is-fedi For a given amount of me'rcury"being circulated ,.the amount of aqueous'soluti onfed in is such' thatbyits'content'of active chlorineandlor'fe'r ricchloride, the iron'cbntent'of the'mercur-yfo'ragiv'en residence time does not fall belowOiQ-l percent weight. If thetreatment is carriedfurtl'ien-there-is' arisk that mercury may be dissolved in-appreciable-quanfi ties. Providedthatthe' iron content-inthe recyclecl' mer cury is notallowed'torise'above aboutOlO I to 0:02pm centby weight,thefelt'formationreferredto above-can'- be'reliably avoided;Thedurationof the" treatment ac-- cording to the'inventionwill"naturally depend on: the amountof'mercury tobet'reated and thusdependsoh' thesize of the electrolytic-cell; lt'amountsas-a rirle'to l to 1 '0percent'of the residence time of the-mercury'in the electrolytic cell;

Chlorine containin'g brine havingthe highest possib'le content ofactivechlorineisadvantageously-used as-tliechlorinewontaining liquidilron chlbride is also used in theform ofa-solution of-the highest possible" concentra' tion'to' which" it'is advantageous to add from ol5 tb 15%byweight'of-HCI. v 1 The following Example illustrates the "invention?EXAMPLE a. 4'cubic meters per'hourof-m'ercur'y (5'4141 tons) iscirculated througha" i kiloampere mercu y; cell. Mercury leavingthedecompjoser'hasan' iron content of'0.0 l 6% by weight. Prior'toreentryinto'the el cittrolytic cell, the mercury is treated with"l6=cubic me ters per hour-of a chlorine-containing sodium chloridebrinehaving a content'of active chlorine 'of340rri'gll covering it therewithin a channel having a IengthjOf S" meters (residence time 22 seconds).

At the end of the channel the mercury-is drawn with an iron content of0.01 2% by weight. The chlorirre' content of the brine has beendecreased to 50 mg of active chlorine per liter.

The cell can be operated for months at a time without} the occurrence ofthe disturbances described bate.

mercuryare achieved under otherwise the same condi ric chloride solutioncontaining hydro-chloric acid'gplil 2) and having acontent of 40 g ofFeper litet;

The aqueous solution contains about l8 g of Fe f per.

liter and 32.88'g of Fe+++ per We claim: I v l. A process for theelectrolysisof an alkali metal chloride by the amalgam method-in a cellhaving a'rnercury cathode, decomposition of the alkali metal alfhalliter after the treatment.

gam thus formed in a decomposer cell vinto mercury and caustic alkalisolution and return of the mercurytb" theelectrolytic cell wherein afterthe-decomposition off the alkali metal amalgam the resultantmercurytreated with'an aqueous solution whichcontains' chlo rine, and/orferric chloride, said treatment'bei'n'gidonf trolled so that the ironcontent of the mercury "is not de' creased below 0.01% by weight. i

b. Thesame results as regards the iron content of tl'i'e 2. A process asclaimed in claim 1 wherein the treatsolution is supplied so that itcovers the mercury. ment is carried out by covering the mercury with a4. A process as set forth in vclaim 1 wherein said aque layer of theaqueous solution.

3. A process as claimed in claim 1 wherein recycled mercury flowsthrough a channel to which the aqueous 5 ous solution containsferricchloride.

t a t e

2. A process as claImed in claim 1 wherein the treatment is carried outby covering the mercury with a layer of the aqueous solution.
 3. Aprocess as claimed in claim 1 wherein recycled mercury flows through achannel to which the aqueous solution is supplied so that it covers themercury.
 4. A process as set forth in claim 1 wherein said aqueoussolution contains ferric chloride.